1. Technical Field
The disclosure relates to a rubber composition used in a tread of a tire and a pneumatic tire using the rubber composition.
2. Related Art
Ice and snow-covered road surface has a friction coefficient remarkably decreased as compared with that of a general road surface, and is slippery. Therefore, in a tread rubber of a tire for winter (winter tire) such as a studless tire or a snow tire, rubber hardness at low temperature is set to a value lower than that of a tire for summer in order to increase ground-contact property on an icy and snowy road surface. Furthermore, to increase on-ice friction force, various methods such as a method of forming a tread rubber using a foamed rubber and a method of blending a hard material such as hollow particles, glass fibers or aluminum whiskers with a rubber are proposed.
For example, JP-A-10-007841 discloses that on-ice frictional performance is improved by a scratch effect by blending vegetable granules obtained by pulverizing seed shells or fruit cores with a rubber. However, the vegetable granules do not have reinforcement, and therefore tend to decrease abrasion resistance performance. For this reason, when the amount of the vegetable granules blended is increased for the purpose of further improving on-ice performance, abrasion resistance performance is deteriorated. Therefore, on-ice performance is desired to be improved while suppressing the deterioration in abrasion resistance performance.
JP-A-2008-115316 discloses that an alkali-treated silk powder together with a filler such as carbon black or silica is blended with a rubber composition for a tire. This document describes that an alkali-treated silk powder is blended with the rubber composition to improve dispersibility of the filler, thereby improving processability, reinforcement and rolling resistance. However, this document is silent on on-ice performance.